The present invention relates to a device for measuring ink density on a printed surface in a running printing machine.
From the German publication No. DE-OS 2023467 a device is known for measuring ink density on a running sheet fed printing press having a measuring head assembly, a light source for illuminating measuring marks on the sheets and a photometer for receiving the emitted light. In order to enable the measurement of the ink density at several locations and transversely to the direction of printing, the measuring head assembly includes either a single measuring head which is designed for adjustment in different measuring positions, or it includes a number of measuring heads corresponding to the number of measured points. The heat effects of the light source on the photometer which might cause impairment in the accuracy of measurement is eliminated by placing the light source and/or the photometer outside the range of the measuring head arrangement and connecting it by means of light conduits.
The construction of a photometer having a response time in the range of about 1 millisecond enables that at a speed of travel of the upper surface of the printed sheet of two meters per second, a minimal length of measuring marks (in the direction of printing) of 5 mm can be used.
The disadvantage of this prior art device is the high technical complexity due to the use of light conduits for eliminating the temperature effects of the light source on the photometer, and in addition the strong dependency of the measuring accuracy on the speed of travel of the upper surface of the printed sheet.